1. Field of the invention:
The present invention relates to a solid electrolytic capacitor.
2. Description of the prior art:
Solid electrolytic capacitors ordinarily possess the structure shown in FIG. 1. This solid electrolytic capacitor is composed of an electrode 2 provided with an anode lead 1; a dielectric oxide film 3, a solid electrolyte layer 4, a carbon layer 5, and a cathode layer 6, which are successively formed on the surface of the electrode 2; a positive terminal 7 connected to the anode lead 1; and a negative terminal 8 connected to the cathode layer 6 by means of an electrically conductive adhesive 9.
A capacitor of this type can be manufactured, for example, in the following manner. First, the insulating plate 210 is attached to the root portion of the tantalum anode lead 1 of the porous tantalum electrode 2, and then the tantalum oxide dielectric film 3 is formed by anodic oxidation on the surface of the porous tantalum electrode 2. Next, the solid electrolyte layer 4, made of manganese dioxide, and the carbon layer 5 are successively formed on the surface of this tantalum oxide dielectric film 3. Then, an electrically conductive coating material containing silver, copper, nickel or carbon powder, etc., a polymer, and an organic solvent is applied and dried to form the cathode layer 6, thus obtaining the capacitor element 200. Next, the tantalum anode lead 1 of this capacitor element 200 is connected by welding to the positive terminal 7, and then, either by soldering or by application of an electrically conductive adhesive 9 containing silver, copper, nickel or carbon powder, etc., a polymer and an organic solvent, followed by drying, the cathode layer 6 is connected to the negative terminal 8. Next, the capacitor element 200 is covered with plastic 10 by the molding method in such a manner that the positive terminal 7 and negative terminal 8 extend in opposite directions to each other. As is required, the terminals 7 and 8 of the capacitor, which extend in opposite directions, are bent toward the inside in a downward direction relative to the body of the capacitor so as to conform with the end and bottom faces of the capacitor.
In the aforementioned conventional type of solid electrolytic capacitor, if either the cathode layer 6 or electrically conductive adhesive 9 are composed of silver powder and a polymer, then migration of silver occurs when the said capacitor is placed under conditions of high temperature and humidity, resulting in short-circuit failures and large current leakage.
On the other hand, if the cathode layer 6 and electrically conductive adhesive 9 are composed of copper powder and a polymer, or composed of nickel powder and a polymer, then, the said copper or nickel does not migrate, as does silver, even under conditions of high temperature and humidity. However, since copper and nickel powder are easily oxidized, the value of tan .delta. increases at high temperatures and high humidity.
If the cathode layer 6 and electrically conductive adhesive 9 are composed of carbon powder and polymer, then migration, as occurs in the case of silver, does not occur even under conditions of high temperature and humidity. However, because the specific resistance of carbon powder is large as compared with that of the aforementioned metal powders, the initial value of tan .delta. is extremely large.